INSTRUMENTATION FOR PREVENTING EXPLOSION AND FIRE (SAFETY)

Naturally, we must safely accomplish measurement, control calculations, and process modulation through adjusting the final element.  Control systems contribute to safe process operation through basic control design, valve failure positions, alarms, safety interlock systems, and pressure relief systems (e.g., Marlin, 2000; AIChE, 1993; Lees, 1996).  This section addresses one important hazardous condition, fire and explosion, that is affected by the design and implementation of control and transmission equipment.  The material in this section is applicable to a wide range of processes and industries using either analog or digital transmission.

SIGNAL TRANSMISSION OF CONTROL SYSTEM

Most process control systems involve a structure of distributed equipment, with sensors and valves at the process equipment and the control calculations and displays located in a remote, centralized facility.  Therefore, values of key variables must be communicated between the sensors, calculations and valves (or other final elements).  Schematics of typical control systems with analog and digital control are given in Figure 4.0.1.

Analyzers

The term analyzer refers to any sensor that measures a physical property of the process material.  This property could relate to purity (e.g., mole % of various components), a basic physical property (e.g., density or viscosity), or an indication of product quality demanded by the customers in the final use of the material (e.g., gasoline octane or fuel heating value).